The present invention relates to a rotary drum shear which cuts a continuously fed long plate such as steel plate or corrugated cardboard at a substantially right angle to the feeding direction of the plate.
A prior art rotary drum shear includes drums which are rotatably disposed at the upper and lower sides and carry straight knives fixedly mounted substantially parallely along an axis of the drums, respectively. The drums are rotated in the opposite direction to each other so that the knives mounted on the drums are engaged with each other once per rotation to cut a long material fed between the drums by the engagement of the knives.
With such a device, when the feeding speed of the material to be always cut changes, it is necessary to increase or decrease the rotational speed of the drums in response to the feeding speed of the material in order to cut the material at a predetermined length exactly. Accordingly, large load is applied to a motor for driving the upper and lower drums and a high-powered motor is required.
As a countermeasure, if load in other portions is lightened, it is possible to use a low-powered motor with low consumption energy. For this purpose, it is considered that a value GD.sup.2 (where G is weight and D is a diameter of the drum) of the drum and a rotating system in the rotary drum shear is reduced. However, since there is a close relation between the GD.sup.2 value and a rigidity, if the GD.sup.2 value is reduced, a flexural rigidity or torsional rigidity of the drum is deceased and the following deficiencies are caused.
While it is required not to enlarge a gap between both the knives in order to cut the material sharply and smoothly, if the rigidity of the drums for supporting the knives is small, the drums are largely bent due to large cutting load generated when cutting and the gap between the knives is enlarged, thereby damaging the cutting quality and the cutting ability greatly.
In order to solve the above problem, there has been recently proposed a method in which curved or spiral knives on the drums are used. According to a theoretical and experimental study by the inventor when the curved knives are used, cutting load can be greatly reduced. In other words, assuming that the cutting load is 1 when a lead angle of the knives that is an angle of the knives with regard to an axial line of the drums is 0.5.degree., the cutting load is about 0.5 when the lead angle is 1.degree., the load is about 0.25 when the angle is 2.degree., the load is about 0.17 when the angle is 3.degree., the load is about 0.13 when the angle is 4.degree., and the load is about 0.1 when the angle is 5.degree.. Thus, the cutting load can be reduced as the lead angle is increased. Accordingly, the rigidity and the GD.sup.2 value of the rotating drums can be reduced, so that the power of the motor can be reduced and energy can be saved.
As described above, by using the spiral knives in the rotary drum shear, there can be expected large merits. However, there is a disadvantage that the material is obliquely cut.
FIG. 1(a) is a view showing a rotary drum shear including spiral knives and the cutting operation thereof. FIG. 1(b) shows a sectional view of the knives in the engagement condition taken along line I--I of FIG. 1(a). In FIG. 1(a), numerals 1 and 2 denote upper and lower drums, respectively, which is rotated at an equal speed and in the opposite direction to each other, numerals 3 and 4 denote knives mounted spirally on the external peripheries of the drums 1 and 2, respectively, and numeral 5 denotes a long material to be cut which is fed between the drums 1 and 2. The material 5 is fed between the drums in a perpendicular direction to rotational axes of the drums 1 and 2. However, the cutting line for the material is inclined by an angle of 90.degree.-.alpha. with regard to the feeding direction as shown in FIG. 1(a).
The reason why the cutting line is inclined is because engagement angles .theta..sub.A and .theta..sub.B at any engagement points in the axial direction of the drums are constant and the knives are always engaged with each other at the same position S since cutting edges of the knives 3 and 4 are held on circles having centers O.sub.A and O.sub.B, respectively, so that it takes a certain time to elapse from a start time to a termination time of the engagement and its time delay causes cutting inclination in the material to be cut.